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BELL SYSTEM TECHNICAL JOURNAL 



plicated that such service would be altogether too costly for com- 

 mercial use; people could not afford to pay for it. 



The Relation Between the Volume and Articulation of 

 Undistorted Speech 



Articulation tests were made upon the high quality telephone 

 system described above when it was set to deliver various intensi- 

 ties from the threshold of audibility to very large values. The 

 results shown as syllable articulation values are given by the curve 



6 5 4 3 



Loudness 



Fig. 4. 



in Fig. 4. The abscissas in this curve represent loudness and are 

 expressed as the natural logarithm of the number of times the speech 

 wave amplitude has been decreased from the initial intensity at 

 ]/^ inch in front of the mouth of the callers. This unit of loudness 

 has never been given a name, and as a matter of convenience in 

 this work it is called a napier. It will be noticed that when the 

 volume is reduced 11}/^ napiers below the initial speech intensity 

 the articulation becomes zero. This point also represents the value 

 at which the speech becomes inaudible and corresponds to approxi- 

 mately 1/1000 dynes per square centimetre pressure variation against 

 the ear drum. In energy units it is a reduction of ten billion times 

 below the initial speech intensity. For very loud initial speech 

 this point is shifted about 1 napier. For purposes of comparison 

 the intensity reductions are also indicated on the loudness axis. 



At 3 napiers below or at about 1/1000 of the initial speech in- 

 tensity the articulation becomes a maximum. Louder speech than 

 this seems to deaden the nerves so that a person makes a less accurate 



